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Subject: Re: [ub] A proposal to define signed overflow submitted?
From: Nick Lewycky (nlewycky_at_[hidden])
Date: 2018-03-15 20:44:52


On Wed, Mar 14, 2018 at 11:06 PM Hyman Rosen <hyman.rosen_at_[hidden]> wrote:

> On Wed, Mar 14, 2018 at 7:10 PM, Nick Lewycky <nlewycky_at_[hidden]> wrote:
>>
>> The compiler is "trusting the programmer" the programmer to never run
>> undefined behaviour.
>>
> The language trusts the programmer in the sense that casts are not checked.
>>
>
> The concept of undefined behavior is erroneous except for things like
> dereferencing pointers that do not point into owned memory, where there
> is literally no meaningful thing that can happen.
>
> I don't see anything in your link that defends the interpretation of
>> "trust the programmer" as
>>
> meaning that compiler should emit obvious assembly. Do you have a
>> supplemental reference?
>>
>
> "Trust the programmer" must mean to make the actions of the compiled code
> follow the actions of the written code, or act as if it does. Anything
> else is
> literally not trusting the programmer.
>
> There are a large number of reasons that a compiler should not emit the
>> obvious operations on ordinary 2's complement hardware, as I have wide
>> freedom to choose the contents of the "...". It ranges from the very
>> obvious ("x = 0;" so constant folding occurs and blocks emission of the
>> negation, or 'x' is never used again so dead code elimination blocks
>> emission of the negation) to the less obvious (~x is used in multiple
>> calculations in different expressions) to the downright hard to reason
>> about.
>>
>
> The compiler still has as-if latitude. As-if transformations maintain the
> actions
> of the written program. Assuming undefined behavior doesn't happen does
> not.
>

As-if transformations maintain the actions of the written program under the
assumption that undefined behaviour does not occur. The two are
fundamentally intertwined.

Here's a few thought experiments:
1. may a compiler replace "x+1+1" with "x+2" assuming x is an int? The two
produce the same results in all cases, even if you choose wrapping,
overflowing, or saturating arithmetic. Suppose you are the vendor of a
compiler and the user complains "I know the add instruction encoding on my
CPU, I crafted a pointer to my instruction stream, I failed to find two
add-with-constant-1 instructions". With what rules would you justify the
transformation "x+1+1" to "x+2"? Currently the answer is that there is no
means to form such a pointer: any attempt to do so necessarily executes UB,
therefore we assume it has not occurred.
2. may a compiler remove "int i = *ptr;" where variable 'i' is subsequently
unused? What if 'ptr' is unaligned or null? Suppose you are the vendor of a
compiler and the user complains, "the underlying hardware traps on
misaligned/null pointers, I expected this execution to trap with the
appropriate type of trap". With what rules would you justify deleting this
dead code? Currently the answer is that null pointer dereference and
misaligned access (well, access of wrong type) are unde
3. may a compiler remove "int i = 0xdeadbeef;" where 'i' is unused? Suppose
you are the vendor of a compiler and the user complains "I put that value
in a stack variable so that I could look through my stack for the magic
number and know where I was in the stack". With what rules would you
justify deleting an unused variable? Currently the answer is that there is
no means to form such a pointer: any attempt to do so necessarily executes
UB, therefore we assume it has not occurred.
3a) A variation on question 3 is to put two variable declarations inside
two arms of an if/else, assigning them to different values. Today every
compiler will allocate space on the stack for all variables once as part of
function prologue, except for functions with VLAs or dynamic alloca calls.
What if the user looks up the stack to determine which side of the if/else
they were being called from?

Today, all these optimizations are justified by the fact that any attempt
to observe the difference before and after optimization must necessarily
execute undefined behaviour, and executing undefined behaviour is a priori
assumed to be impossible.

It's possible that there's another way to show that these and other
optimizations are valid, but I would need to see a proposal on how to do
that. I've tried to create one myself and failed. Frankly, I'm starting to
believe that our current situation with UB really is the best possible
design choice.

The other reason to do something other than emit the "obvious operations"
>> is to detect unintended overflow. For one large industry example Android
>> security is deploying unsigned integer overflow checking:
>> https://android-developers.googleblog.com/2016/05/hardening-media-stack.html
>> . Instead of overflow, their compiler emits an overflow check and trap
>> instruction.
>>
>
> That's fine. Ada generated exceptions on overflow and other subtype
> constraint
> violations from its inception. It simply means that the underlying
> computer isn't a
> 2's-complement arithmetic machine, and programs written for 2's-complement
> arithmetic won't work there. C and C++ programs do not have to be
> portable to
> be valid. It's going to be up to the users of the non-2's-complement
> platform to
> decide if it fits their needs.
>
> What semantics does "issue the underlying machine instructions and return
>> whatever result they provide" have?
>>
>
> The compiler has a mapping from abstract machine to its target computer,
> whereby it represents program objects as machine objects. The target
> computer
> has its own semantics for arithmetic operations. By far the most common
> semantics for machine integer arithmetic is fixed-width 2's-complement, and
> when that's the case, that's what the quoted phrase means - do the
> arithmetic
> in 2's-complement and return the result.
>
> That is what the standard explicitly requires everywhere for atomic ints,
> so I don't
> see why you would find this surprising or difficult.
>
>
>> What about template non-type arguments? Constant expression evaluation?
>> Used in the size of an array?
>>
>
> What is difficult to understand? For those things, the compiler's target
> platform
> is an artificial target platform embedded within the compiler itself. For
> ease of
> programmer understanding, that platform should behave in the same way as
> the
> actual target platform.
>

The difficulty is that the compiler would then need to have a processor
emulator for the target platform. This is not presently how compilers work.
It's possible that this emulator could in fact be made simple by dealing
only with the subset of cases that the compiler itself relies on. Presently
compilers simulate execution on top of a single abstract machine described
in the language specification, and produce programs with equivalent
behaviour to the abstract machine. The details of the underlying machine
*largely* don't trickle upwards to affect evaluation in the abstract
machine (except when it does, like in sizeof).

What if the compiler uses "whatever the machine does" when determining the
>> size of an array as part of compilation, then we copy the resulting program
>> to a different computer that claims to have the same ISA, but where
>> "whatever that machine does" happens to be different. (This situation would
>> be surprising for integer arithmetic, but did occur for x86 floating point
>> arithmetic.) Are you okay with the compiler evaluating one size for an
>> array at compile time, but calculating a different size for the array at
>> run time?
>>
>
> As I said, it would be extremely confusing for the compiler to do that, so
> it shouldn't.
>
> But C and C++ already have the maximum possible stupidity of allowing
> floating-point
> expressions to be calculated at different precisions, so that it is
> literally possible for the
> expression a + b == a + b to be false when the operands are normal
> floating-point
> values. And everyone seems to accept that with equanimity. Optimization,
> naturally.
>

What if I tell you, for the sake of argument, that compilers today are
>> already following
>>
> your proposed rule: since you didn't specify which instructions to issue,
>> you have no
>>
> standing to complain about the instructions the compiler chose.
>>
>
> You don't get to tell me whether or not I have standing.
>

I'm not clever enough to see what possible argument you could make. Maybe
you can suggest something? The setup is that you've written a program and
the compiler emits assembly you didn't consider "the obvious lowering". You
complain to the compiler authors and they say "yes, this is the obvious
lowering". Now how would you proceed? There's no specification to reference
to determine whether the compiler did emit the "obvious lowering" or
not, that's
what I meant by "not having standing".

  I'm going to complain about
> this forever, so that all the people who made the bad decisions will feel
> bad about
> themselves, and not sleep at night because of all the grief they've caused
> programmers.
>

I agree! But I arrived at the opposite conclusion. I wish compilers
exploited undefined behaviour to the fullest extent, and I wish they had
started doing more of it longer ago. If only the optimizer broke people's
programs the first time they ran them, programmers would have noticed the
failure sooner and not proceeded with their buggy code. The sooner the
optimizers get better at breaking buggy code in mean and nasty ways, the
sooner we'll stop writing even more code with these kinds of bugs in it. I
would rather deal with that pain now so that we have a better world
tomorrow.

You also don't get to be immunized from critics who weren't there when
> decisions were
> made. Learning from bad decisions of the past helps prevent bad decisions
> in the future,
> for those willing to learn.
>
> Can you fix this without adding a listing of CPU instructions to the
>> language standard
>>
> and without fully defining it?
>>
>
> I don't need to. The C++ Standard already specifies that atomic integers
> obey the rules
> of 2's-complement arithmetic, and it doesn't spell out what those rules
> are.
>

I would consider it shall "obey the rules of 2's complement arithmetic" to
be fully defined, as 1's complement machines would have to simulate 2's
complement arithmetic. (Offhand, I think this is what happens with atomics
today; 1's complement machines are expected to use a library call to
implement the atomic and ensure 2's complement?)

Indirecting a pointer should mean "refer to
>>> the memory pointed to as if there is an object there of the pointer
>>> type" and
>>> should be undefined only if the pointer does not point to correctly
>>> aligned
>>> memory owned by the (entire) program. And so on.
>>>
>>
>> Suppose I have a function with two local variables, "int x, y;" and I
>> take &x, I can use
>>
> x[1] (or x[-1]) to make changes to 'y'?
>>
>
> Yes, provided you know that the memory layout corresponds to that.
>
> And similarly, I can construct a pointer to stack variables in another
>> call stack frame
>>
> whose address was never taken? As long as I can cast the right integer
>> to a pointer?
>>
>
> Yes.
>
> Given these rules, when would it be valid to move automatic local
>> variables into registers?
>>
>
> Always. If a variable is not in memory, then a pointer can't be made to
> point to it,
> no matter how the pointer is manipulated. As I said, you must know the
> memory
> layout in order to do tricks with pointers.
>

The as-if rule only applies if the program can not tell the different
between the before program and the after program. If you can form pointers
this way, the program now has a way to observe the difference, revoking the
compiler's permission to perform the optimization. With these rules, the
compiler may no longer remove stack variables from the stack, or reorder
them, etc. I don't think that's the language we want. Do you have a
suggestion on how to both permit the construction of such pointers and also
permit this optimization?

Only when there are no opaque pointers used and no opaque functions called?
>>
>
> No, always. In fact, that's why we had the register keyword. That told
> the compiler
> that the address of such variables could never be taken.
>

I don't think C++ users want a language where copies of all automatic
storage variables must be present on the stack.

There might be a market for a different language which really does behave
that way, but I should point out that it's been tried. The proposal for
Friendly C: https://blog.regehr.org/archives/1180 and later The Problem
with Friendly C: https://blog.regehr.org/archives/1287 .

Nick





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